Prepare Cupcakes: On waxed paper, combine flour, cocoa, baking soda, and 1/2 teaspoon salt. In 2-cup liquid measuring cup, mix sour cream, milk, and remaining vanilla; set aside. In large bowl, with mixer on low speed, beat granulated sugar and remaining 8 tablespoons butter just until blended. Increase speed to high; beat 1 minute. Reduce speed to low; add eggs, 1 at a time, beating well after each addition. Beat 1 to 2 minutes more or until light and fluffy. Add flour mixture alternately with sour cream mixture, beginning and ending with flour mixture. Beat just until ingredients are combined, occasionally scraping bowl with rubber spatula.

Spoon 1 heaping measuring tablespoon batter into each muffin-pan cup. Drop 1 peanut butter ball in center of each cup and top with another heaping tablespoon batter. Bake cupcakes 25 to 28 minutes or until toothpick inserted in cupcake comes out clean. Immediately remove cupcakes from pans and cool completely on wire rack.